Roderick W. Tillman1
(1) Consulting Geologist/Stratigrapher, Tulsa, OK
ABSTRACT: An Incised-shelf, Paleovalley-fill, Tidal, Point-bar Reservoir Analog
The Newcastle (Muddy) Sandstone in the town of Newcastle, Wyoming is an excellent reservoir analog for tidal paleovalley-fill sandstone reservoirs. Tidal accretion-bar sandstones (point-bars) were the initial, and primary, filling deposits in a regionally extensive, incised valley, eroded at least 80 ft into the underlying shelfal-marine Skull Creek Shale. Within the paleovalley there are up to 43 ft of continuous, vertically stacked tidal bars, which thin and thicken abruptly. The total sandstone thickneses in three representative outcrop measured sections, within a half mile of one another, vary significantly (43 ft, 30 ft and 66 ft), and there is no evidence that the bars, which include the type section of the Newcastle Sandstone, are connected.
Detailed mini-permeameter surveys of the outcrops show cycles of vertically varying, abrupt changes in permeability on a less than 0.1 meter scale. Gamma-ray surveys of the outcrops sharply define sandstones and allow correlation with fields within a mile of the outcrop. Recently completed paired-horizontal wells, utilizing steam-heating of oils may bring production even closer to the outcrop and provide a mini-boom to the area.
The tidal bars have an external geometry similar to fluvial point bars, however, the internal sedimentary structures and baffles to flow are quite different. The sandstones are deposited as a stacked sequence of 0.5-3.0 ft thick, very low-angle dipping, parallel-bedded and parallel-laminated, to very low-amplitude rippled sets. The suspension deposited laminae are marked by fining upwards from fine sandstone to shaley-carbonaceous siltstones.
AAPG Search and Discovery Article #90906©2001 AAPG Annual Convention, Denver, Colorado